1. Field of the Invention
This invention relates to the filtering of metallic debris out of a solution and, more particularly, to the magnetic filtering of particle debris out of work-functioning baths.
2. Description of the Prior Art
In many industrial processes, involving the use of work-functioning baths or slurries, there has often been a need to remove undesired foreign matter that tends to accumulate therein. In particular, it is often desired to remove metallic material, such as magnetically attracted ferrous debris, typically in the form of minute particles, from the desired composition of a given bath. Through such a filtering operation, a given bath may often be purged of detrimental debris to such an extent that the useful life thereof is greatly increased.
The presence of metallic particle debris, particularly if allowed to cumulatively build-up in a given bath, may often lead to a number of serious consequences. For example, such debris may often seriously impair the surface condition, or at a minimum impair the appearance, of a given article immersed in or otherwise brought into contact with the bath. In addition, depending on the nature of the bath, such debris may lead to the decomposition of, and/or in some other way effect a reduction in the strength or concentration of, one or more constitutents of the bath, such as through an oxidation process precipitated by the presence of metallic debris.
The removal of magnetic or metallic debris from a bath has become of particular importance in the wet chemistry cleaning of ceramic substrate-supported silicon integrated circuits (SIC's). More specifically, in the manufacture of such circuits it is necessary to clean the metallized surface of each ceramic substrate not only prior to the thermocompression bonding of SIC devices to the substrate, but prior to the encapsulation of the composite circuit within a protective coating.
During both of these cleaning operations, generally using a common bath consisting of a given concentration of boiling hydrogen peroxide, an appreciable amount of the magnetic particles that form the necessary magnetic ink-like coatings applied to the SIC devices, to facilitate the automated handling thereof, are dislodged. These particles, as well as other minute metallic particles in the form of undesired debris, if not in some way removed, will cumulatively build-up in the bath. The particles that are not chemically altered tend to re-deposit on the surfaces of the ceramic substrate, and the ones that adhere to the metallized circuit side of the substrate often prove to be particularly detrimental, as they can readily cause troublesome circuit shorts. Unfortunately, such shorts can occur not only immediately after a given cleaning operation, but subsequently as a result of certain particles becoming dislodged and acquiring short-circuit positions during further manufacturing operations performed on the circuits, particularly prior to and during their encapsulation within a protective coating.
With respect to those ferrous type particles that become oxidized, they normally ultimately disintegrate in the bath. Such a particle debris oxidation-disintegration process within a hydrogen peroxide bath can disadvantageously accelerate the decomposition of H.sub.2 O.sub.2 therein and, thus, necessitate the rejuvenation of the bath more frequently by additions of concentrated hydrogen peroxide. In high volume integrated circuit wet chemistry cleaning operations, such frequent replacement of hydrogen peroxide, as well as periodic replacement of the entire bath when contaminated beyond the limits for effective practical use, can be costly as well as time consuming.
Heretofore, any magnetic particle debris that disadvantageously remained on the substrates, was generally removed, to the extent possibly, by a deionized water spray or rinse. Unfortunately, these procedures would not always dislodge all of the tenaciously adhering particles from the substrate and, as a result, many of such particles would remain on the metallized circuit side of the substrate causing short circuits.
Even when such particle debris did not cause short circuits, it would produce an undesirable visual surface appearance, and often present difficulties in bonding the silicon devices to the substrate.
One technique employed heretofore to remove magnetic, as distinguished from non-magnetic, material in general as it is formed within a slurry or bath is disclosed in U.S. Pat. No. 3,712,472, of E. G. Elliott. In one embodiment therein, a magnetic material transporting apparatus is partially submerged within a container-confined slurry or bath composed of both matnetic and non-magnetic material. The transporting apparatus comprises a pair of spaced, parallel manifolds of non-magnetic material, with a plurality of spaced, parallel tubes also of non-magnetic material extending between the manifolds. Each end of each tube in the one embodiment is connected to a different one of the manifolds, with a magnet functioning as a piston being slideably confined in each of the tubes. A fluidic system is coupled to both manifolds, with a selectively operable valve employed to cause fluid flow in either direction through the tubes so as to drive the magnet-functioning pistons in the direction of fluid flow.
With one of the manifolds, and a substantial portion of each tube being continuously submerged in the bath, while the other manifold and the remaining length of each tube remain outside the bath, magnetic material within the bath is attracted against each tube in the immediate area of the associated submerged magnet, and carried by magnetic attraction therealong to a non-magnetic barrier positioned outside the container. The barrier is apertured so as to allow each tube to pass in close-fit relationship therethrough. Thus, as the fluid-driven magnet in each tube moves through the barrier, the magnetic material removed from the container by the magnet is forced off of the tube and collected. In another embodiment disclosed in the same prior art reference, the piston-functioning magnets are driven along the respective tubes by rotatable shafts connected thereto.
While such prior magnetic apparatus is capable of removing at least a certain amount of metallic debris from a slurry or bath, the magnets disadvantageously are neither disposed in an array so as to extend over the major cross-sectional area of the container, nor are they adapted for simultaneous reciprocal displacement from the top to the bottom and back to the top of a given bath. Only in this way can the entire bath be visualized as subdivided into discrete cubicle zones that are at least periodically exposed to, and screened by, an array of frame-supported magnets.
The prior art apparatus in question also occupies an appreciable area of a given tank, thus minimizing the useful area, as well as depth, within which a given piece part (or parts) may be immersed within a given bath. In addition, the need to utilize an external power source to displace the singular magnets along the respective support tubes appreciably adds to the cost and complexity of the magnetic transport system.